On Android, according to the Coalition Against Stalkerware, there are over 1 million victims of deliberately placed spyware on an unlocked device by a malicious user close to the victim every year.
#2 is WAY more likely than #1. And that's on Android which still has some protections even with a sideloaded APK (deeply nested, but still detectable if you look at the right settings panels).
As for #3; the point is that it's a virus. You start with a webkit bug, you get into kernel from there (sometimes happens); but this time, instead of a software update fixing it, your device is owned forever. Literally cannot be trusted again without a full DFU wipe.
And where are the stats for people running their own firmware and are not running stalkerware for comparison? You don’t need firmware access to install malware on Android, so how many of stalkerware victims actually would have been saved by a locked bootloader?
The entirety of GrapheneOS is about 200K downloads per update. Malicious use therefore is roughly 5-1.
> You don’t need firmware access to install malware on Android, so how many of stalkerware victims actually would have been saved by a locked bootloader?
With a locked bootloader, the underlying OS is intact, meaning that the privileges of the spyware (if you look in the right settings panel) can easily be detected, revoked, and removed. If the OS could be tampered with, you bet your wallet the spyware would immediately patch the settings system, and the OS as a whole, to hide all traces.
Assuming that we accept your premise that the most popular custom firmware for Android is stalkerware (I don’t). This is of course, a firmware level malware, which of course acts as a rootkit and is fully undetectable. How did the coalition against stalkerware, pray tell, manage to detect such an undetectable firmware level rootkit on over 1 million Android devices?
This assumes a high level of technical skill and effort on the part of the stalkerware author, and ignores the unlocked bootloader scare screen most devices display.
If someone brought me a device they suspected was compromised and it had an unlocked bootloader and they didn't know what an unlocked bootloader, custom ROM, or root was, I'd assume a high probability the OS is malicious.
LineageOS alone has around 4 million active users. So malicious use is at most 1:4, not 5:1.